1. Field of the Invention
This invention relates to an improved strained superlattice semiconductor photodetector having a side contact structure.
2. Prior Art
While PIN type semiconductor photodetectors having pn junctions have been used in practical applications, Metal-Semiconductor-Metal (MSM) type semiconductor photodetectors realized by utilizing Schottky junctions have been attracting researchers' attention.
Since an MSM type semiconductor photodetector comprising a pair of oppositely disposed electrodes on a crystal surface is particularly suited for use in a highly integrated circuit such as an OEIC (opto-electronic integrated circuit), a number of studies have been made in this technological field.
While an MSM type semiconductor photodetector has advantages of having a very small capacity and being structurally simple and therefore capable of being produced with a reduced number of manufacturing steps, it has certain technological problems to be solved before it can make a feasible opto-electronic device, including a relatively large dark current that has to be reduced and a relatively long rise time of the responding pulse that needs to be curtailed in order to achieve a fast pulse response.
A long rise time of the responding pulse of an MSM type semiconductor photodetector is attributable to the fact that holes generated as a result of optical absorption move rather slowly so that they tend to get to the cathode considerably long after the end of light exposure.
The use of a strained superlattice has been proposed as means for accelerating holes in order to eliminate the above identified technological problems.
This proposal is based on the fact that, if a semiconductor photodetector has a strained superlattice structure, the effective mass of holes in the strained superlattice layer having intra-planar compressive strains of the device is reduced along intra-planar directions to enhance the mobility of holes, when compared with a device having no strained structure.
FIG. 4 illustrates a typical MSM type semiconductor photodetector realized by utilizing a strained superlattice structure.
Referring to FIG. 4, a nondoped InP buffer layer 12, a photodetective layer 13 having a strained superlattice structure and comprising an InGaAs compressive strain layer and an InGaAs tensile strain layer, a non-doped InAlAs layer and Ti/Pt/Au electrodes are sequentially laid on a (100) Fe-doped InP substrate 11.
An MSM type semiconductor photodetector having a strained superlattice structure as shown in FIG. 4 is accompanied by the following problems when it is used for a practical application.
(1) After moving in parallel with an intra-planar direction in the strained superlattice structure, carriers move perpendicularly relative to the strained superlattice until they get to an electrode. PA1 (2) Carriers are generated by incident light in both the compressive and tensile strained superlattice layers and caused to move toward an electrode in the two strained superlattice layers. While holes behave as lightweight holes in the compressive strained superlattice layer, they remain as heavyweight holes in the tensile strained superlattice layer so that the speed at which they move cannot be significantly improved as a whole. PA1 (3) An increased leak current appears from any reversely biased Schottky junctions to increase the dark current of the device because of a low Schottky barrier between each electrode and a semiconductor on the InP substrate of the device.
In other words, their responsiveness is degraded as they pass over a barrier layer of the strained superlattice (and they give rise to a pile up phenomenon at a heterobarrier).